Pyrolytic deposition of silicon nitride films

ABSTRACT

Silicon nitride films are prepared on graphite and other suitable substrates by the vapor phase pyrolysis of a silicon halide-ammonia adduct and preferably a silicon tetrafluorideammonia adduct (SiF4.2NH3).

2v vva /\1\ J'MJJIPTLJ 3K Sestrich 1 51 Jan. 25, 1972 i s41 PY-ROLYTICDEPOSITION OF SILICO 3,485,666, 12/1969 Sterling et a1. .;..1 17/106 xIDE FILMS 3,503,798 5/1970 Satoshi Yoshioka et al ..1 17/106 X [72)inventor: Donald E. Sestrich, Monroeville, Pa. Primary Examingr-AlfredL. Leavitt Assislan! Examiner-Wm. E. Ball [73] Asslgnee' 2:32: 2?Electric Corponnon Pm- Attomey-F. Shapoe and CL. Menzemer [22 Filed: vFeb. 10, 1969 21 Appl. No.: 798,056 I [57] ABSTRACT [52] [1.8. CI.....ll7/106 R, l17/D1G. 12 Silicon nitride films are prepared ongraphite and other suita- [51] Int. Cl ..C23c 11/14 ble substrates bythe va or hase 'yyolysis of a silicon halide- 1 2 P P P [58] Field ofSearch ..l 17/ 106, DlG. 12 ainmonih adduct and referabl a silicontetrafluoride-am- P Y monia adduct (SH-1.21911 {156] References Cited f9 1 Claims, 2 Drawing Figures 9 7; UNITED STATES PATENTS 3,226,19412/1965 Kuntz 17/106 X K C x 2 :1. X 1 7 Ansolvl VACUUM K 5 SOURCE OF 52FLOW ULTRFIJAZPURE METER 1 v 1" i u PATENTFU JANZSIQIZ mwhwE INVENTORDonald E.-Sestrich BY ,GKWM

wmna m.53 m0 mUmDOm ATTORNEY PYROLYTIC DEPOSITION OF SILICON NITRIDEFILMS BACKGROUND OF THE INVENTION l. Field of the invention Thisinvention is related to the pyrolytic deposition of silicon nitridefilms on graphite and other suitable substrates.

2. Description of the Prior Art Tungsten, molybdenum, and graphite areamong the materials used inhigh temperature electrical applicationsbecause of their favorable physical and electrical properties. Theirhigh melting points, high strength, and electrical conductivity atelevated temperatures, as well as their relatively low coefficients ofthennalexpansion, have made them useful as lamp til ts, furnace heatingelements, and as electrodes or electrical co tacts.

The use of tungsten, molybdenum, and graphite is limited to either avacuum or an inert atmosphere environment since conductors made of anyof these materials are subject to rapid oxidation in the presence of airor water vapor at high temperatures. Their utility could be greatlyextended if an oxidation resistant coatingcould be developed which wouldserve as an electrical insulator as well as to protect the conductorfrom oxidation in air at high temperatures.

Silicon nitride (Si N is an inert refractory material similar to aceramic in electrical resistivity and oxidation resistance. Adherentfilms of silicon nitride on graphite or other suitable substrates wouldbe useful as wire insulation or in the fabrication of capacitors and semi on net evrces.

Urban E. Kuntz in his U.S. Pat. N573I226J94 describes how a film ofsilicon nitride can be formed by the pyrolysis of a reactant gas mixtureof a silicon halide and ammonia above the hot surface of a suitablesubstrate. The two principal drawbacks of this process are (1) one mustalways maintain a balance of the flow of reactant gases and theirreactant gas mixture otherwise the silicon nitride which is formed willnot deposit properly'on the heated surface of the substrate, and (2) theimpinging reactant gases can react anywhere within the apparatus and mayeven plug up this system thereby slow- DESCRlPTlON OF THE INVENTION I Itis preferred that the starting material for the making of siliconnitride (Si Nfl in accordance with this invention be a solid material.The solid material should have stoichiometric,

or almost stoichiometric, ratios of silicon and nitrogen. An excess ofnitrogen is preferred in the compound if a stoichiometric ratio is notobtainable. A suitable solid material is an adduct ofa silicon halideand ammonia, (SiX,-2NH;).

The silicon halide-ammonia adduct materials upon standing in air reactwith the moisture in the air to'yield (NH).SiX,, Nl-LX, SiO,, and NILX,where X is either fluoride, chlorine, bromine, or iodine. The mostunstable silicon halide-ammonia adduct materials in the presence ofmoisture in the air are silicon tetrabromide-ammonia and silicontetraiodide-ammonia. These two adduct materials are quite unstable andreact readily with the moisture of the air.

7 A more stable silicon halide-ammonia adduct material is silicontetrachloride-ammonia. However, the reaction of the silicontetrachloride-ammonia adduct and the moisture of the air still occurstoo rapidly to make it a preferred starting material.

The most stable of the silicon halide-ammonia adduct materials issilicon tetrafluoride-ammonia adduct SiFrZNH 300C. The adduct uponsublimation and recrystallization in a vacuum 115 C. produces pure SiF-2NH a solid starting material for making of silicon nitride films.

With reference to FIG. 1 there is shown apparatus 100 suitable forpreparing silicon tetrafluoride-ammonia adduct material. The apparatus100 is comprised of a first conduit material on substrates from a sourcematerial of silicon tetrafluoride-ammonia adduct.

Other objects of the invention will,.in part, be obvious and will, inpart, appear hereinafter.

SUMMARY OF THE INVENTION the vaporized adduct to the heated substrate,the silicon halide and ammonia of the adduct reacting in the presence ofthe heated substrate to produce silicon nitride which deposits on theheated substrate.

DRAWINGS For a better understanding of the nature and objects of theinvention, reference should be had to the-following drawings in which:

112 connectedby a suitable gastight means 114 to a quartz reactorchamber 102 disposed in a flask 104 and having the capability of beingevacuated. A suitable flask 104 into which the chamber 102 may bedisposed is a Dewar'Flask. The chamber 102 is connected by a gastightmeans 116 to a first trap 106. The first trap 106 is connected by agastight means 118 to a second conduit 120 which in turn is connected bya gastight means 122 to a second trap 108. The second trap 108 isconnected by a gastightmeans 124 to a mercury-bubbler 110. A shutoffvalve 126 in an outlet tube 128 from the mercury bubbler 110 regulatesany gas flow from the bubbler 110.

The first conduit 112 has a plurality of shutoff valves 130 to regulategas flows from a plurality of gas sources and to'regulate a gas flow toand from the conduit 112 immediately before the gas sources. The secondconduit 120 has a gate valve 132 to regulate any gas flow to the secondtrap 108, and a gate valve 134 regulating a gas flow to and from theconduit 120 to a third conduit 136. The third conduit 136 has a gatevalve 138 regulating a gas flow to a vacuum producing means (not shown)and a gate valve 140 regulating a flow of a gas for purging theapparatus 100.

The apparatus 100 is evacuated and flushed with argon to purge thechamber 102 of air. Silicon tetrafluoride is disposed in the quartzreaction chamber 102. Liquid nitrogen is poured into the'flask 104 andfreezes the silicon tetrafluoride in the reaction chamber 102 at atemperature of l 95.8 C.

The reaction chamber 102 is purged of any gaseous silicon tetrafluoridewhich may be present after freezing by use of ultrapure nitrogen gas.The ultrapure nitrogen gas employed in the process of this invention isa substantially oxygen and water free gas having an oxygen content of0.9molar parts per million and a water content of 0.53 molar parts permillion. Anhydrous ammonia, in a sufl'rcient amount in excess of thatrequired for chemically reacting with the silicon tetrafluoride,

Through manipulation of the flask 104, the frozen silicon tetrafluorideand the frozen anhydrous ammonia are permitted to warm up and to reactwith each other in a controlled manner. The chemical reaction resultingin the formation of the adduct is accompanied by the evolution of aconsiderable amount of heat thereby causing rapid volatilization of thefrozen components. This rapid volatilization of the frozen componentsproduces the adduct SiF.-2NH both by the gas phase in the reactionchamber 102 principally and by sublimation of the SiFyZNl-L, material inthe reaction chamber 102 resulting in the deposition of the adduct SiF,-ZNH, in one or more traps 106 and 108 installed in the apparatusdownstream from the reaction chamber.

Upon reaching room temperature, the apparatus 100 is opened and thesilicon tetrafluoride-ammonia adduct SiF,-2N H;,, which is a solid whiteproduct, is removed. The adduct, as produced, is impure and possiblycontains (NHQ SiF NH F and (NHJSiFgNHf. The impure adduct is placed inapparatus suitable for the purification of the adduct by sublimation.The impure adduct is sublimed at atemperature of from.-

75 to l 15C. in a continuouslypumpedvacuum of at least 5 micronspressure. The purified SiFfiNl'l adduct is'collected' on split Pyrexsleeves placed in the sublimation apparatus.

Referring now to H6. 2, there is shown apparatus suitable for the vaporgrowth of silicon nitride (Si m) films on a as alumina. A suitable means22 for heating the adduct 18 in a the boat 20 is provided. Such suitablemeans 22 include a resistance heater placed outside the tube 12 in thevicinity of the boat 20.

Downstream from the adduct 18 material, but within the chamber 12, asuitable substrate 24 to grow the silicon nitride on is mounted onalumina rods 27 placed on an open ended I alumina boat 28.Radiofrequency coils 30 are disposed about the outside of the reactionchamber 12 in the vicinity of the substrate 24 to heat the substrate 24to a desired elevated temperature. The substrate 24 consists of amaterial selected from the group consisting of molybdenum, silicon,tungsten, tantalum, and graphite.

Downstream from the reaction chamber 12 is disposed a 3- way valve 32.Outlet 34 is connected to manifold 36 to enable one to evacuate thesystem of the apparatus 10 through line 38 and shutoff valve 40 or toflush the system with argon through Thesource material is heated to atemperature of from 60 to 174 C. When the source material 18 has reachedits operating temperature, the process is continued for the length oftime required to grow the necessary thickness of silicon nitride on thesubstrate 24. When the process has continued for the necessary time. theheating means 22 is turned off first, allowing the source material 18 tocool first. The radiofrequency coils are then turned off and the coatedsubstrate 24 is allowed to cool to room temperature in the stream ofultrapure nitrogen. 1

It is preferred, that when the substrate 24 is made of graphite, thatthe graphite substrate 24 be prebak-ed in a vacuum at a temperature inexcess of 1 ,400 C. for a period of l hour.

The following examples illustrate the teachings of this invention: 2

EXAMPLE 1 Silicon tetrafluoride-ammonia (SiF.2NH adduct was made by themethod described in this invention. The reaction apparatus for producingthe adduct was evacuated and flushed with'a'rgon gas 3 times.Approximately 25 cubic centimeters of SiF, was introduced into thereaction chamber and frozen at the temperature of liquid nitrogen-byimmersing the reaction chamber in liquid nitrogen in a Dewar Flask. Thereaction apparatus was purged again with nitrogen gas which was asubstantially water and oxygen free gas being used to remove any tracesof gaseous SiF Forty cubic centimeters of anhydrous ammonia, slightlydiluted with the ultrapure nitrogen remaining in the system afterpurging, was frozen on top of the frozen SiF... The 40 cubic centimetersof anhydrous ammonia was sufficient to provide an excess of NH; for thelater chemical reaction.

The Dewar Flask was manipulated to permit the Sil? and the N l-l to warmup and to react with each other in a con trolled manner. The adductSiFrZNH was formed mainly in the reaction chamber although some wasformed in the traps as well. Upon reaching room temperature, theapparatus was opened in air and the adduct removed.

The adduct SiF '2Nl-l was transferred to a slightly inclined Pyrex glasstube,-lined with split glass sleeves and sublimed at 82fi C. in acontinuously pumped vacuum of 5 microns pressure for a period of 15hours. Upon completion of the sublimation process it was found that 42.7percent of the starting material remained as a residue.The analyticalresults for line 42 and shutoff valve 44. Outlet 46 of the 3-way valve32 is connected by line 48 to at least one trap 50 and the mercurybubbler l6. 7

The apparatus 10 is evacuated to at least 60 microns of pressure, andflushed with argon gas, other suitable gases for flushing the apparatus10 are nitrogen and ammonia. This is repeated several times. Theapparatus 10 is then purged further with a substantially water andoxygen free gas such, for example, as substantially water and oxygenfree nitrogen gas, from the source 14 entering the chamber 12 throughfirst a metering valve 52, thence throdgh a flow meter.54 and ashut offvalve 56. The substantially water and oxygen free gas is employed as acarrier gas. Other suitable carrier gases are sub stantially water andoxygen free argon and ammonia.

The flow of nitrogen gas is continued as the substrate 24'is started tobe heated to an elevated temperature. The gas flow rate is from 100 to250 cubic centimeters per minute, with a preferred gas flow rate ofapproximately 250 cubic centimeters per minute. The substrate 24 isheated to a temperature of from 850 to l,385 C.'When the substrate 24has reached its desired elevated temperature, the heating of the sourcematerial i8 is commenced.

the sublimated adduct were as follows:

k N Si 5 F Calculated for SiF,-2NH, 20.28 20.34 $5.04 By chemicalanalysis l9.l 20.] 56.]

X-ray analysis of the sublimated adduct generally agreed the residue tocontain (NHQ SiF unsublimed SiF,-,2NH and two phases not identifiable.Approximately 3.0 to 3.5 grams of the sublimated adduct was placed in analumina boat in the far end of the apparatus 10 as shown. The reactionchamber was 30mm. 0.0. x 22 inches and was in series with a tank ofultrapure nitrogen gas and a mercury bubbler. 1

A graphite substrate was polished'with 4/0 paper and was prebaked atl,4()0:25 C. for a period of 1 hour in a vacuum. The graphite substratewas mounted on alumina rods placed in holes drilled in the substrate andin turn was mounted on an open ended alumina-boat at the position of theexternal coils of the radio frequency generator.

The system was evacuated to 60 micro torrs of mercury and flushed withargon three times. Substantially oxygen and water free nitrogen gasflowing at a gas flow rate of 250 cubic centimetersper minute-for frhour purged the system. The subsublimed adduct SiF.-2NH was heated to170%5" C. Substantially oxygen and water free nitrogen gas flowing at agas flow rate of l cubic centimeters per minute was first passed overthe adduct and then over the heated substrate. The gas flow continuedfor 3 hours. The resistance heater was turned off I first and removedthereby cooling the sublimated adduct. The

. RF generator-was shut offand the substrate cooled. The

nitrogen flow was maintained and the substrate cooled to i roomtemperature. The apparatus was opened and the substrate removed forexamination.

A transparent film of silicon nitride has been deposited on thesubstratefl'he material was electrically insulating when tested with aresistance meter. The deposited film was found to be silicon nitride (SiN EXAMPLE H The process of example I was followed except that thesubstrate temperature was l,325i:25 C., the temperature of the adductwas 100 C., the gas flow rate was 250 cubic centimeters per minute andthe process was continued for 5 hours.

An insulating film as verified by a resistance meter had been grown onthe substrate. The film was so adherent to the substrate that thesubstrate was partially destroyed to obtain the grown material foranalysis. Analysis proved the material to be silicon nitride (Si NEXAMPLE I" The process of example I was followed except that the substrate comprised a molylgtleri u n rod polished with 4/0 polishing paperand washed ih organic solvents, the substrate temperature was 1,1501225C., the adduct temperature was 9315 C., the gas flow rate was 250 cubiccentimeters per minute and the process was continued for 6 hours.

An insulating film as verified by a resistance meter had been grown onthe molybdenum substrate. The insulating film was analyzed and found tobe silicon nitride (Si N Further analysis of the silicon nitride filmsgrown by X-ray diffraction analysis techniques shows the adherenttransparent films to be amorphous. 1

l claim as my invention:

1. A process for growing silicon nitride on a substrate comprising thesteps of:

a. disposing a silicon halide-ammonia adduct in one end of a reactionchamber;

b. disposing a substrate at the opposite end of the same reactionchamber;

c. evacuating the reaction chamber;

d. causing a substantially oxygen and water free carrier gas to flowthrough the reaction chamber, first over and about the adduct and thenceover and about the substrate; Y

e. heating the substrate;

KT. vaporizing the adduct whereby the carrier gas transports'wthevaporized adduct to the heated substrate, the silicon halide and ammoniareacting in the presence of the heated substrate to produce siliconnitride which deposits l on the heated substrate. H) The process ofclaim 1 in which thesubstrate is a material selected from the groupconsist ing of molybdenum, silicon, tungsten, tantalum, and graphite. 3.The process of claim 1 in which the carrier gas is a gas selected fromthe group consisting of nitrogen, argon and ammonia.

4. The process of claim 2 including the step of purging the reactionchamber with argon gas prior to cans ing the flow of the substantiallyoxygen and water free carrier gas through the reaction chamber. 5. Theprocess as defined in claim 2 in which the silicon halide-ammonia adductissilicon tetrafluorideammonia. 6. The process of claim 2 In whichammonia adduct is heated to a temperature of from 60 to I 174 C. and thesubstrateis'heated to a temperatureof from 850 to 1,385" C. p

9. The process of claim 7 in which thecarrier gas hasa gas flow rate offrom .l 00 to 250 cubic centimeters per minute. 10. The process of claim8 in which the carrier gas has a gas flow rate of cubic centimeters perminute for at least the period of depositing silicon nitride on theheated substrate. 11. The process of claim 9 in which the carrier gas isnitrogen having a gas flow rate of 250 cubic centimeters per minute forat least the period of depositing silicon nitride on the heatedsubstrate.

1' t I! 8 t

2. The process of claim 1 in which the substrate is a material selectedfrom the group consisting of molybdenum, silicon, tungsten, tantalum,and graphite.
 3. The process of claim 1 in which the carrier gas is agas selected from the group consisting of nitrogen, argon and ammonia.4. The process of claim 2 including the step of purging the reactionchamber with argon gas prior to causing the flow of the substantiallyoxygen and water free carrier gas through the reaction chamber.
 5. Theprocess as defined in claim 2 in which the silicon halide-ammonia adductis silicon tetrafluoride-ammonia.
 6. The process of claim 2 in which thesubstrate material is graphite and including the process step of bakingthe graphite substrate in a vacuum at a temperature in excess of 1,400*C. for a period of 1 hour prior to disposing the substrate in thereaction chamber.
 7. The process as defined in claim 4 including thestep of cooling the substrate with the silicon nitride deposited thereonin the carrier gas.
 8. The process of claim 6 in which the silicontetrafluoride-ammonia adduct is heated to a temperature of from 60* to174* C. and the substrate is heated to a temperature of from 850* to 1,385* C.
 9. The process of claim 7 in which the carrier gas has a gasflow rate of from 100 to 250 cubic centimeters per minute.
 10. Theprocess of claim 8 in which the carrier gas has a gas flow rate of 100cubic centimeters per minute for at least the period of depositingsilicon nitride on the heated substrate.
 11. The process of claim 9 inwhich the carrier gas is nitrogen having a gas flow rate of 250 cubiccentimeters per minute for at least the period of depositing siliconnitride on the heated substrate.